1. Field of the Invention
The present invention relates to a thermal printing method and a thermal printer, wherein heating data for a thermal head is corrected with regard to resistance variation between heating elements of the thermal head, and heat accumulation in the thermal head. More particularly, the present invention relates to a thermal printer and a thermal printing method, wherein heating data is corrected after being converted into data representative of a driving or power conduction time of each individual heating element.
2. Background Arts
Sublimation transfer type or ink transfer type thermal printers and thermosensitive type or direct imaging type thermal printers have been known as printers which is capable of recording each dot at a different density to print an image with gradation. The former heats an ink film with a thermal head to transfer ink onto a recording sheet. The latter directly heats with a thermal head a thermosensitive recording sheet having a thermosensitive coloring layer per one color, to make the layer develop color. The thermal heads have a large number of heating elements aligned in a main scan direction. While the thermal head is moved relative to a recording sheet in a subscan direction perpendicular to the main scan direction, the heating elements are driven to record an image on a recording sheet one line after another. Each heating element is heated to record one dot in a pixel, i.e. one virtual square segment of the recording sheet. The heating energy of each heating element is controlled in accordance with heating data of each pixel, to change the density of the dot in accordance with a tonal level designated by the heating data. Hereinafter, heating data of each pixel will be referred to as pixel heating data.
For example, as shown in FIG. 10, when printing one dot on the thermosensitive recording sheet, one heating element first performs a bias heating by applying a bias heat energy Eb to the recording sheet to heat it up to such a degree above which a desired thermosensitive coloring layer begins to develop a color. Thus, the amount of bias heat energy Eb is determined by the heat sensitivity of the coloring layer. After this bias heating, a gradation heating for causing the coloring layer to color at a different density is performed by applying a different amount of gradation heat energy Eg to the recording sheet. With these bias heating and gradation heating, one pixel is colored to form one dot. The bias heat energy Eb and the gradation heat energy Eg are controlled in accordance with bias data and image data respectively. Accordingly, heating data for the thermosensitive type thermal printer consists of bias data and image data. On the other hand, heating data for the ink transfer type requires only to image data.
To control the amount of heat energy generated from the individual heating element in accordance with the pixel heating data assigned to that heating element, the amount of electric power supplied to each heating element is controlled. However, as shown for example in FIG. 10, the recording density is not proportional to the gradation heat energy Eg applied to the thermosensitive recording sheet. That is, the actual gradation of tonal levels is non-linear with respect to the gradation heat energy Eg, while the tonal levels represented by the image data are based on a linear gradation. Therefore, the pixel heating data is converted into time data representative of a driving or power conduction time through the heating element necessary for recording a dot at the designated density. This is because the heat energy generated from the heating element increases in direct proportion to the power conduction time through that heating element, as the heating elements are constructed of resistors. Conventionally, power conduction time through the heating element is controlled for each pixel by changing the time duration of one continuous driving or the number of times of intermittent driving in accordance with the heating data of that pixel.
It is also known in the field of thermal printing that the recording density is affected by heat energy accumulated in the individual heating elements as well as in the whole thermal head. The results are undesirable variation in density, inadequate contrast, so-called shading etc. As the heating elements are resistors, a variation in the resistance also results an unexpected variation in density. To prevent these problems, the heating data is corrected so as to eliminate the influence of heat accumulation and the resistance variation between the heating elements. Hereinafter, these correction processes of the heating data will be referred to as the correction due to the thermal head. Many methods for the correction due to the thermal head have been suggested, for example, in U.S. Pat. Nos. 5,528,276 and 5,608,333, and U.S. patent application Ser. Nos. 08/698,695 and 08/768,942.
Because of the non-linear relationship between the image data and the gradation heat energy, when correcting the heating data, for example, such that consequent gradation heat energy values are corrected by the same rate compared with the respective original values, it is necessary to multiply the image data of each tonal level by a different correction coefficient or factor. These different correction coefficients must be calculated from the value of image data, i.e. the original heat energy value, and the correction value of heat energy, in accordance with formulas that are determined based on the non-linear recording density curve of the thermosensitive coloring layer relating to heat energy, such as shown in FIG. 10. Such a calculation is complicated. Since this calculation should be done for each individual pixel of each line, and one line consists of a large number of pixels, it takes a certainly long data processing time or needs highly complicated operation circuits. In result, the total printing time is elongated, or the production cost of the thermal printer is increased.